This invention relates to a random access memory which utilizes a ferroelectric capacitor as its basic storage unit, and, more particularly, to such a memory wherein the ferroelectric capacitor is isolated from the data bitline capacitances by a buffer amplifier.
Many military and some civilian systems require data storage capability in a random access memory that is nonvolatile and resistant to data loss due to disruptive events. A nonvolatile memory retains the stored data even when power is lost or turned off. Such nonvolatile memories are often based upon physical changes that occur in a memory element when data is stored, which physical changes are not reversed or randomized upon the loss of power.
Ferroelectric materials have been successfully developed for use in nonvolatile memories. A ferroelectric material is one whose physical state changes upon the application of an electrical field, in a manner analogous with the change undergone by ferromagnetic materials to which a magnetic field is applied. The ferroelectric material has the advantages that its physical state can be controlled by the application of a voltage rather than a magnetic field or current, a measurable physical state is retained after a power loss, and memory elements can be constructed by microelectronics fabrication techniques.
To utilize this capability, the ferroelectric material is typically made the dielectric of a capacitor, so that the physical state of the ferroelectric material may be either changed or sensed by applying a voltage to the plates of the ferroelectric capacitor. Electronic circuitry is provided to write data into the ferroelectric capacitor by altering its physical state using an applied voltage and to read the stored data by sensing the physical state of the ferroelectric capacitor.
There are two classes of read circuitry used in ferroelectric memories. In a destructive readout (DRO), the physical state of the capacitor is erased and the stored data must be rewritten as part of each memory cycle. Accordingly, there is a short but finite time period during each read cycle where the data is stored only in the electronics of the device. In the other type of readout, nondestructive readout (NDRO), the stored data is not erased during reading. There are two types of NDRO ferroelectric memories. In the first type of NDRO ferroelectric memory, the memory will recover without loss of data from several, typically 2-20, power failures during reading without restoring. In the second type of NDRO ferroelectric memory, the number of reads without the requirement of restoring is very large or infinite, so that a restore during each read is not required.
Although existing nondestructive readout ferroelectric memories are fully operable, their size is large in comparison with that of conventional destructive readout, non-volatile random access memory devices. The number of memory units that may be packaged within a preselected size is therefore relatively small, limiting the total storage capacity of the memory. The large size does not pose a serious limitation for some applications where only a limited amount of information must be stored in a nonvolatile manner. The large size does limit the use of the ferroelectric memory in other applications.
There is a need for a nondestructive readout ferroelectric memory wherein the size of each memory unit is reduced, so that the total memory storage capacity may be increased. The present invention fulfills this need, and further provides related advantages.